Heretofore, there have been reported many graded-index mediums made of glass such as described in the Japanese Patent Publication No. 47-816/1972. The glass mediums, however, have unfortunately been manufactured with low productivity, high cost, and are commonly weak when bent during practical uses.
In addition to the glass mediums, several plastic graded-index mediums have been proposed. In particular, the synthetic resin optical transmission medium described in the Japanese Patent Publication No. 47-28059/1972 uses rather simple methods for forming index distribution, and is quite available for manufacturing. That is to say, the transmission medium as described in the Japanese Patent Publication No. 47-28059/1972 is provided an index distribution through the steps comprising admixing two or more polymers having different refractive indices and different solubilities with respect to the specific solvent when compared to each other, molding the thus obtained mixture to form a rod or fiber, and dipping thus obtained rod or fiber in said specific solvent to elute a portion of the polymers in a different proportion from the proportion of mixing of the polymers. The thus produced graded-index medium, however, has several drawbacks as follows.
(i) In general, the mixture prepared by direct blending of polymers having different refractive indices has an unsteady index distribution and has a transparency lower than the respective polymers. Therefore, such a mixture of polymers can only be used in an extremely short light path medium such as in a lens, but cannot be used in a long light path medium such as in a optical fiber, because of much light scattering along the long path.
(ii) The respective polymers are required to be transparent and must be selected from a small extent of materials, so that it is difficult to find out an adequate combination of polymers to obtain low transmission loss mediums.
(iii) In many cases, the transparent polymers are amorphous with poor mechanical properties. Therefore, the substance obtained by the polymer blend has little practical value in manufacturing fibrous mediums such as optical fibers required to have good properties when bent action as well as adequate softness.